1. FIELD OF THE INVENTION
The present invention relates to an apparatus for measuring the amount of air taken into the internal combustion engine of an automobile or the like, or more in particular to an air intake apparatus for the apparatus for measuring the amount of air taken into the internal combustion engine, comprising a heating resistor of a constant-temperature type disposed in a bypass of the venturi.
2. DESCRIPTION OF THE PRIOR ART
Some conventional air intake apparatuses have a heating resistor disposed directly in the main air path as disclosed by U.S. Pat. Nos. 3,824,966 and 3,992,940. The heating resistor of these apparatuses is degenerated in quality or deformed by a high-temperature flame or a high-pressure blast caused by a back fire, so that the resistance value thereof is reduced, thus adversely affecting the flow rate measuring accuracy. In extreme cases, the heating resistor is broken.
In order to overcome this disadvantage, other conventional air intake apparatuses comprise a bypass of the venturi, in which a heating resistor is disposed, as disclosed in Japanese Patent Laid-Open No. 113432/77. In this type of air intake apparatus, the heating resistor, which is not exposed directly to the flame or blast of a back fire, is not deformed or degenerated in quality which otherwise might change the characteristics thereof. In spite of this advantage, it has the problem points as mentioned below.
(1) The ends of the heating resistor are connected to supporting conductors which are connected to a drive circuit for the heating resistor by lead wire. The variation of the lead wires leads to the variation of the wiring resistance. Also, the variation in the amount of solder used on the lead wire causes the variation in the measurement accuracy of the air flow rate.
(2) The air introduction port of the conventional main air path and bypass are in many cases formed of aluminum or the like by die casting and therefore is easily affected by the thermal change of the internal combustion engine. Specifically, during the driving of an automobile, the temperature of the part of the automobile provided with the heating resistor gradually rises, thus changing the temperature of the heating resistor making up a temperature sensor through the lead wire or the supporting conductors. This reduces the measuring accuracy.
(3) The heating resistor is often used in juxtaposition with a resistor for air temperature compensation. The conventional heating resistors are comparatively large in size and therefore the two resistors are arranged longitudinally of the air flow in the bypass, namely, along the direction of the air flow in the bypass. As a result, the air heated by the heating resistor disposed upstream comes into contact with the compensating resistor disposed downstream, thereby leading to a reduced measurement accuracy. In order to overcome this problem, other conventional air intake apparatuses have the two resistors staggered, in which case the lengths of the two supporting wires are different, with the result that the thermal effect of the air introduction port is different, thereby reducing the measuring accuracy.
Means for preventing the thermal effect on the resistors is disclosed in the above-quoted U.S. Pat. No. 3,824,966. The resistor of this patent, however, does not take the back fire into consideration as it is disposed in the main air path.